There have recently been proposals to allow access to the features and services provided by cellular networks, such as GSM and UMTS networks, other than by accessing those networks in the conventional manner. In this regard, the conventional manner is by signalling between a mobile terminal and a conventional base station (macro base station) that has a connection to Base Station Controller (BSC) which in turn has a connection to a Mobile Switching Centre (MSC). The BSC is a component of the network which controls radio resource management, in order to ensure that a user is connected to the best base station in terms of signal strength or quality.
It has been proposed to increase network capacity by providing additional special base stations, for example at a subscriber's home or office. Many different names have been given to these special base stations such as home base stations, access points (APs), home access points (HAPs), IMS home base stations, home Node Bs (HNBs), pico-base stations, pico-cells and femto-cells, but all names refer to the same apparatus. For ease of reference, these base stations will be referred to here as home base stations or Home Node Bs (HNBs). Home base stations provide short range, localized cellular telecommunications coverage, and are typically purchased by, or rented to, a subscriber to be installed in their house or business premises. In this regard it is to be appreciated that the term “home” is not intended to be limited solely to use in residential premises.
It has also been proposed to use home base stations in the Long Term Evolution (LTE) telecommunications network currently being developed, but not yet implemented. LTE is likely to be the next network implementation after 3G (and 3G's incremental improvements, often referred to as 3.5G).
These home base stations may be dedicated network access points, or may be enhanced wireless internet hubs (i.e. providing wireless internet access, as well as wireless telecommunications network access). The range of home base stations is significantly smaller than macro base stations, typically providing coverage to a range much less than 100 meters.
An advantage of introducing home base stations in existing telecommunications networks is that, where sufficient numbers of them are implemented, the power level of the macro coverage could be reduced, due to a lower demand for the macro-base stations. Power reductions of course result in energy and financial savings, for instance due to less spectrum being required and also less hardware.
A further advantage of using a home base stations connected to the core network via an IP network is that existing broadband Digital Subscriber Line (DSL) connections can be used to link mobile terminals with the network core without using the capacity of the radio access network or transmission network of a mobile telecommunications network. In other words, the home base station can be integrated into a DSL modem/router so as to use DSL connections to backhaul the traffic to the communication network.
A still further advantage is that home base stations are able to provide “mobile” network access to areas where there is no macro radio access network coverage. For example, a home base station could provide 3G coverage in an area where there is no macro 3G coverage at all, perhaps only macro GSM coverage. The use of home base stations as an additional or alternative means for accessing the network therefore advantageously increases the network capacity and coverage.
Further additional challenges arise in implementing these home base stations as conventional base stations, in view of their ability to be installed and provisioned by the end consumer. Issues regarding the security of the home base station need to be addressed, particularly in view of possible modifications by an owner. For instance, one problem is that the owner of a home base station may seek to modify it in order to receive calls not intended for them, but, for instance, intended for another person or other entity that the owner wishes to eavesdrop or spy on, such as a famous person. A further problem is that the owner may wish to avoid call charges and seek to modify their home base station so that calls put through it are charged to another person's account.
A further problem relates to the fact that IMS Home Node Bs utilise SIP signalling. IMS (IP Multimedia Subsystem) is an architectural framework for delivering Internet Protocol (IP) multimedia services in mobile networks. However, at present it is not possible to move calls from legacy mobile terminals making MSC domain calls into the IMS domain via IMS Home Node Bs as the signalling used by such legacy mobile terminals is not compatible with SIP signalling.
Additionally, the current procedures used at the radio interface for communicating with mobile terminals are in accordance with 3GPP TS 24.008, which at present is not wholly compatible with SIP signalling. There is therefore a need to improve the interworking between mobile terminals and IMS Home Node Bs in order to allow users of such terminals to be able to fully utilise the IMS Home Node Bs.
In particular, a problem with this compatibility lies in the fact that the IMS Home Node B needs to register each mobile terminal it is serving with the IMS system, which is a SIP server. Currently the IMS registration from the IMS HNB is performed at the same time as a UMTS location area (LA)/Routing Area (RA) update (or a GSM LA/RA update in a Dual Transfer Mode (DTM) capable GSM system). In this regard, as part of the R′99 paging coordination functionality used in such LA/RA updates, the applicable MSC/SGSN sends the mobile terminal's IMSI to the HNB, and the I-INB then uses this IMSI to register the terminal at the HNB's IP address/name.
However, it is to be appreciated that HNBs contain some RNC functionality, and so, when UEs attach via a HNB, the HNB stores the UE's IMSI/TMSI along with the wireless ciphering and integrity protection keys. These keys will remain valid as long as the UE stays in its current Location Area (LA)/Routing Area (RA). Therefore, from the HNB's viewpoint, these keys can then be used for subsequent communications with the UE without contacting the MSC/SGSN, until a point in time when the UE subsequently performs a LA update (LAU)/RA update (RAU). However, the proper functioning of this procedure also requires the HNB to be in a LA/RA different to that of its surrounding/overlaying cells (being either other “home cells” or cells from the traditional PLMN). This is because where a UE moves between base stations within a LA/RA, there are certain situations where the keys and/or the UE's Temporary Mobile Subscriber Identity (TMSI) change, but, since there will be no LAU/RAU, the HNB will not be notified of the changes. Therefore, if the UE subsequently returns to the HNB, having stayed with the LA/RA but changing its TMSI and/or security keys since it was last attached to the HNB, the HNB will not be aware of the new cipher keys. Therefore the HNB is likely to attempt communication with the UE using its saved, but not current, cipher keys. Its only once this has occurred, and signalling resources wasted, that it may become evident to the HNB that a LAU/RAU is required.
Therefore, ideally each HNB is allocated its own LA/RA for this technique to function correctly, so that the UE always performs a LAU/RAU when it moves away from the HNB. However, in order to limit the signalling load on the core network, it is desirable for a Home Node B to be in the same LA and/or RA as its surrounding cells, and so this is not really feasible. Therefore there is a need to ensure the HNB has the most up-to-date information, but without unduly increasing the signalling load.
A still further problem in relation to the compatibility between SIP signalling and 3GPP TS 24.008 is that whilst entry into an IMS HNB causes IMS registration, idle mode movement away from the HNB does not lead to deregistration in most situations. This is problematic, particularly in terms of the increased signalling load that is likely to occur when attempting to route calls/communications towards a mobile terminal that has in fact moved away.
There is therefore a need to overcome and/or alleviate at least one of the problems of the prior art.